a) Field of the Invention
The present invention relates to a zoom lens system, and more specifically to a compact zoom lens system for use with cameras, etc. employing electronic image pickup devices.
b) Description of the Prior Art
Under the recent trend to develop compacter video cameras for domestic use, various attempts are made to design compacter imaging lens systems. In order to design compacter imaging lens systems, efforts are made to shorten total lengths of the lens systems and reduce diameters of front lens components.
In the attempts to design compacter zoom lens systems, field angles of the lens systems are narrowed and aperture ratios thereof are lowered to F/2.0 from F/1.4 which was the conventional main specification item.
The conventional zoom lens system for video cameras consists, in the order from the object side, of a first lens unit having a positive refractive power, a second lens unit having a negative refractive power and movable along the optical axis for variation of focal length, a third lens unit having a negative or positive refractive power and movable along the optical axis for compensating the deviation of the image point caused by the variation of focal length, and a fourth lens unit which is designed as a relay lens unit having an imaging function and always kept stationary, and comprises an aperture stop fixed between the third lens unit and the fourth lens unit or in the fourth lens unit. The conventional zoom lens system having the composition described above requires wide airspaces to be reserved between the front lens component and the aperture stop, and has an entrance pupil located deep in the lens system, whereby the front lens component inevitably has a large diameter. As a zoom lens system which has the above-described composition but is designed so as to use a front lens component having a small diameter, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-54213. This zoom lens system uses a first lens unit and a second lens unit which are thinner so as to locate the entrance pupil shallow in the lens system, thereby being compatible with a front lens component having a smaller diameter. Further, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-287810 uses first through third lens units which are composed of lens elements in a number as small as possible and selects a refractive power distribution which permits effectively utilizing the airspaces varied for variation of focal length so that the entrance pupil thereof is located shallow in the lens system and a small front lens component is compatible with the lens system.
Furthermore, the displacement of the first lens unit along the optical axis for focusing can be mentioned as another factor for increasing the diameter of the front lens component. That is to say, when the first lens unit is displaced toward the object side for focusing, the entrance pupil of the lens system is located deep in the lens system, thereby making it necessary to enlarge the diameter of the front lens component. As a conventional example which corrects the defect of the enlargement of the front lens component for the reason described above, there is known, for example, the zoom lens system disclosed by Japanese Patent Kokai Publication No. 60-184221. This zoom lens system is designed so as to perform focusing by moving the lens unit arranged after the aperture stop, thereby preventing the front lens component from having a large diameter. However, even this conventional zoom lens system is insufficient in the reduction of the diameter of the front lens component.
Moreover, there are known zoom lens systems which omit the third lens unit having the function to compensate for the deviation of the image point caused by the variation of focal length (the function as a variator) by sharing the function of the variator among all or some of the lens units which are arranged after the aperture stop and have the function of the relay lens system, thereby narrowing the airspaces between the aperture stop and the front lens component arranged on the object side thereof, and allowing to use a front lens component having smaller diameters. These zoom lens systems are those disclosed by Japanese Patents Kokoku Publication No. Sho 47-42175, Kokoku Publication No. Sho 57-12125, Kokai Publication No. Sho 63-123009 and so on. Especially by sharing the function of the compensator among all of the lens units arranged after the aperture stop (the rear subsystem) and selecting such a refractive power distribution as to displace the rear subsystem for zooming in the direction opposite to the moving direction of the second lens unit G.sub.2 having the negative refractive power, it is possible to impart a vari-focal function also to this rear subsystem and shorten the displacement distance for zooming of the second lens unit having the vari-focal function. When the displacement distance of the second lens unit G.sub.2 is shortened as described above, the airspaces can be reduced between the aperture stop and the front lens component, thereby making it possible to reduce the diameter of the front lens component. However, the zoom lens system which is designed so as to displace the rear subsystem arranged after the aperture stop for zooming is undesirable since it allows aberrations, especially coma and spherical aberration, to remain in uniform tendencies over the entire vari-focal range. For this reason, there are proposed zoom lens systems, as exemplified by that disclosed by Japanese Patent Kokoku Publication No. Sho 57-12125, which are adapted so as to correct aberrations favorably over the entire vari-focal range by arranging, after the third lens unit, an additional lens unit which has a fixed location, a weak refractive power and a purpose of correcting aberrations.
However, the zoom lens system disclosed by Japanese Patent Kokoku Publication No. Sho 57-121215 does not contribute so much for reduction of the diameter of the front lens component since it comprises an aperture stop which is arranged between the third lens unit and the fourth lens unit.
In addition, it is possible to reduce the diameter of the front lens component of the zoom lens system disclosed by Japanese Patent Kokoku Publication No. Sho 58-32364 when the aperture stop comprised therein is located at the position specified for the aperture stop arranged in the lens system disclosed by Japanese Patent Kokoku Publication No. Sho 47-42175. As an example of such a zoom lens system, there are known the lens systems, for example, that disclosed by Japanese Patent Kokai Publication No. Sho 61-296317.
This conventional example consists, in the order from the object side, of a focusing lens unit, a negative variator lens unit, a positive variator lens unit and a fixed lens unit, is adapted so as to perform variation of focal length while maintaining the image point thereof at a constant location by displacing both the variator lens units in the directions reverse to each other, and comprises an aperture stop which is fixed at a middle position between the positions of the variators when there are located closest to each other. In contrast to the lens system disclosed by Japanese Patent Kokoku Publication No. Sho 62-54213 and the similar ones wherein aperture stops can be located only at positions on the image side of the airspaces reserved between the third lens units and the fourth lens units, said conventional example comprises the aperture stop between the variators (the second lens unit and the thrid lens unit), i.e., on the object side of the airspace reserved between the third lens unit and the fourth lens unit, and therefore can have an entrance pupil located closer to the lens unit, thereby permitting reducing the diameter of the front lens component. Though said conventional example permits reducing the diameter of the front lens component, it has a defect to enlarge the diameter of the rear lens component. This is because the entrance pupil is brought closer to the first surface of the lens system but the exit pupil is brought farther from the final surface of the lens system by shifting the aperture stop toward the object side. Moreover, it cannot be said that this conventional example permits sufficiently reducing the diameter of the front lens component and has a total length sufficiently short.
In the future where video cameras will be compacter and image pickup devices will be smaller, it will be necessary to design a compacter imaging lens system. For compacter designs of imaging lens system, it is insufficient to make the lens system smaller as a whole in proportion to reduction of an imaging surface since marginal portions of lens elements cannot be thinned extremely and airspaces between lens elements cannot be narrowed extremely from the viewpoint of practical manufacturing and assembly of the lens elements. For this reason, reduction of the diameter of the front lens component poses a problem very important for lens design.